In network maintenance, it is critical to localise faults accurately. Faults can occur for a variety of reasons, but precise localisation of these faults has often been a problem. For example, remote testing might suggest a likely fault in a joint somewhere in a PSTN line, but without a precise localisation method, an engineer may have to spend considerable time performing manual testing on the line at various points in order to locate the fault. Accurate fault location information can enable engineers to spend their resources more efficiently, allowing them to put more effort into fixing the faults instead of locating the faults.
A conventional public telecommunications network can generally be described as having two main parts: a core network, and an access network. The access network is the part of the network that extends from the customer premises or terminal equipment to the local exchange. The core network provides services to customers, handles call routing, and other main functions.
In the access network, a communications line is formed of a pair of copper or aluminium wires. Typically each wire passes through a series of nodes between the local exchange and the terminal equipment. Examples of such nodes include cable segments, primary cross-connection points, secondary cross-connection points, distribution points, and joints.
More recently, optical fibres have been used in access networks to replace copper wires, with both copper and optical fibres being used together. Where a communications line consists of an optical fibre, the line will typically pass through several nodes between the local exchange and the terminal equipment. At each node, the incoming fibre from the local exchange is routed, and may be split into a group of outgoing fibres which branch out in various directions. The last part of the circuit to the terminal equipment may still be carried by a pair of copper wires.
Since the components of the access network are those which are most exposed to the elements, this part of the public telecommunications network is particularly prone to faults. In a conventional access network, examples of such faults include: disconnection faults, where the communications line is interrupted between the local exchange and the terminal equipment; short circuit faults, for example electrical leakage between the two wires of a line pair; and earth leakage faults, for example electrical leakage between one of the wires and earth. The causes of the faults include physical damage to a node as well as leakage of water into a node.
To enable a network provider to remedy faults rapidly, local exchanges are provided with line testing apparatus which may be used to test each line. Such tests can be used to determine the approximate location of the fault between the local exchange and the terminal equipment.
EP1269728 describes a system and method for testing a telecommunications network for a fault. Changes in capacitance values of a line are measured and compared to a threshold. If the change exceeds the threshold, then a fault is signalled. A known capacitance length ratio is used to estimate the distance to the fault.